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(CBS) — Ghost voters are legendary in Chicago politics.

But now, the CBS 2 Investigators have learned thousands of ghosts have been getting more than $6 million in property tax exemptions in Cook County for decades.

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The Cook County Assessor’s Office is cracking down on this type of tax  fraud. Under a $1.3 million contract with Lexis Nexis, Social Security lists of dead people were cross-matched with those getting senior exemptions and senior property tax freezes.

They got 4,000 hits.

The Chicago gravestones of John and Stanislawa Iwanyszyn show the couple died in 2005, but their names have been used to get senior exemptions and freezes at the Northwest Side home where they used to live — for a total savings of $24,151 in property taxes for the current property owner.

It’s just one of thousands of improper exemptions Assessor Joseph Berrios is removing from the records.

So far, they’ve caught 3,809 erroneous senior exemptions that collectively gave property owners $6.2 million in tax breaks they did not deserve.

It’s a cliché that dead people used to vote in Cook County, but Berrios says dead people getting tax exemptions will be stopped immediately.

“All these individuals that are doing this should be put on notice: We will find you, and when we do find you we’re going to put liens on you property if you don’t pay the money that you owe us,” Berrios says.

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Property owners over 65 must apply every year for a senior exemption and cannot qualify for a senior freeze if their household income is over $55,000.

They have to sign the forms “and be on record that you’re still alive,” the assessor says.

Armed with a new law he proposed to the legislature, Berrios can go back four years to collect undeserved tax breaks plus interest and penalties.

That is what happened to Margaret Kloniecka, the owner of a home in the 1500 block of North Wood St. in Wicker Park.

She got $34,597 in tax breaks over the last four years. With interest and penalties, she recently paid back $61,003.

Kloniecka inherited the property from her cousin, who died in 2001. She says any underpayments were due to a “mistake” by a lawyer.

Combined with purging the property tax rolls of improper homeowner exemptions, Berrios predicts the total savings for the past year will be $12 million to $14 million.

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The question is whether assessors in other counties will push for the same kind of program.